The primary goal of this project is to design and implement a multi-institutional, open-label phase I/II study of hydroxurea (HU) therapy in children with sickle cell disease. To achieve this main goal, the project has four specific aims: 1) To determine if HU will elevate the concentration of fetal hemoglobin, hemtocrit and red cell mean corpsucular volume in children with sickle cell anemia between the ages of 5 and 18 years. 2) To determine if the therapeutic dose of HU in pediatric patients is similar to that in adults. 3) To determine if the hematologic and renal toxicities in children are similar to those in adults. 4) To determine if there are adverse effects on growth in children taking HU.